1. Field of the Invention
This invention relates to 3rd Generation (3G) mobile communication system, especially to a method for providing a Multimedia Broadcast/Multicast Service.
2. Description of the Related Art
The MBMS is a new service under standardization by 3rd Generation Mobile Communication System Partnership Project. The MBMS service is a unidirectional point-to-multipoint (p-t-m) (i.e. multimedia data sent from a single data source are transferred to multiple users through network transmission) service. The most remarkable feature of the service is that it can make use of radio resources and network resources efficiently. MBMS service is mainly used in wireless communication network system, e.g. Wideband Code-Division Multiple Access system (WCDMA), Global System of Mobile Communication (GSM), etc. MBMS service data transfer basically includes following several steps, i.e. data source transmission, middle network transmission, destination cell on-the-air transmission and user receiving. FIG. 1 is a logical figure for network devices of a radio communication system that can provide the MBMS service, in which the MBMS actually makes use of General Packet Radio Data Service (hereinafter referred to as GPRS) as core transmission network. As shown in FIG. 1, a Broadcast and Multicast Service Center 70 (hereinafter referred to as BM-SC) is a data source for MBMS data transmission. A Gateway GPRS Supporting Node 60 (hereinafter referred to as GGSN) is used to connect the GRPS network with an external network, such as INTERNET. In the MBMS service, the Gateway GPRS Supporting Node, connects to the BM-SC and sends the MBMS data to a specific Service GPRS Supporting Node 50 (hereinafter referred to as SGSN). The SGSN is used to perform access control on UE as well as mobility management, and sends the MBMS data from the GGSN to a specific Radio Network Controller 30 (hereinafter referred to as RNC) at the same time. The RNC is used to control a group of base stations like station 23 and 24 and sends multimedia data to specific base stations like station 23 and 24. The base station 23 establishes an air common channel 11 for the MBMS service of cell 21 under the control of the RNC. The base station 24 establishes an air common channel 12 for the MBMS service of cell 22 under the control of the RNC and a terminal User Equipment 10 (hereinafter referred to as UE) is a terminal equipment for MBMS data reception.
When implementing a point-to-point communication between the UE and the network, a dedicated channel will be established by the network for the UE and the radio resource occupied by this channel can not be shared by any other UE but exclusively possessed by the UE in the communication process. In order to make use of the radio resource and the network resource effectively when the MBMS service is provided via the network, a common channel is adopted in the air interface for communicating between several UEs simultaneously. After the UE obtains the resource configuration of the common channel, it receives radio signals via the network so as to gain the MBMS service. However, an obvious feature different from the dedicated channel is that no common channel supports soft handover. Soft handover means that the user can simultaneously receive signals from several cells and then merge them to obtain better quality of signal and lower loss rate of data. For the UE 10 in FIG. 1, if the transmission channels are established for it in the cell 21 and cell 22 by the network, it can simultaneously receive signals from the cell 21 and cell 22 on the premise that the cell 21 is adjacent to cell 22.
To support soft handover, a certain time delay must be satisfied to the signals received by a physical layer of the UE. For instance, in current WCDMA system, the time delay is required to be controlled within ±148 chip, i.e., 0.03854 milliseconds. Whether this requirement is met or not is controlled by the service RNC of the UE as shown in FIG. 2. Reference numeral 21 denotes cell 1, and 22 denotes cell 2 in FIG. 1. 201 shows a timing relationship corresponding to the cell 21, and 202 shows a timing relationship corresponding to the cell 22. In the following, definitions of respective clock in the WCDMA system are described:
BFN: a frame number of the Node B, each frame is 10 milliseconds long, and the frame number repeats from 0 to 4095.
SFN: a system frame number of the cell under the control of the Node B, each frame is 10 milliseconds long, and the frame number repeats from 0 to 4095. The time difference between the SFN and BFN is one Tcell, which values from 0 to 9 with the granularity of 256 chip (about 1/15 milliseconds), i.e., the Tcell values 0, 2/15, 3/15, 4/15, 5/15, 6/15, 7/15, 8/15, 9/15 milliseconds.
CFN: CEN involves a type of channel through which the UE receives signals, if the channel is a common one, it equals SFN modulo 256, and if the channel is a dedicated one, it equals SFN subtracted by a frame offset modulo 256. A CFN1 and CFN2 in FIG. 2 are corresponding to the common channel, while a CFN1′ and CFN2′ are corresponding to the dedicated channel. The only difference consists in that the frame offset of the dedicated channel in 201 is zero while the frame offset of dedicated channel in 202 is 6. In an actual system, whether the dedicated channel or the common channel is in service, a chip-level time difference within 0˜149 256-chips may exist between a start point of the channel's frame and that of the SFN's frames. To be simplified, as shown in FIG. 2, it is assumed that Dcoff is the chip offset of two cells 21 and 22 corresponding to the dedicated channel, and Soff is the chip offset of the two cells 21 and 22 corresponding to the common channel.
Since there is no device such as a GPS is applied in the WCDMA system for all network elements in clock adjusting, it is possible that the clock of every network element be different. Moreover, a deviation may happen to every clock's frequency. The RNC has its own clock for time control, and the Node B also has its own clock for time control. However, since all resource configurations of the Node B are under the control of the RNC, and the time for data sending is also managed by the RNC, it is necessary to make the RNC have some knowledge to the clock's characteristics of the Node B. FIG. 3 illustrates a method of node synchronization between the RNC and the Node B. The clock of the RNC is denoted by RFN, which is the frame number of the RNC side, each frame is 10 milliseconds long and the frame number repeats from 0 to 4095. The time within a cycle can be denoted as from 0 to 40959.875 with the granularity of 0.125 milliseconds, which equals to the length of 480 chip. The RNC sends a downlink node synchronization message to the Node B, which contains time t1 when the message is sent. After the Node B receives that message, it records time t2 when the message is received, then sends an uplink node synchronization message to the RNC, which contains the time t1, t2 and t3. Here, t3 is time when the Node B sends the uplink node synchronization message. After the RNC receives the message, it records time t4 when the message is received. Till now, the node synchronization process between the RNC and the Node B completes. According to the four parameters t1, t2, t3 and t4, a data transmission delay between the RNC and the Node B can be calculated by the RNC, and a time corresponding relationship between the RNC and the Node B can also be set forth. It is assumed that a RTD denotes the time delay between the RNC and the Node B, RTD=(t4−t1−t3+t2)/2. The time corresponding relationship between the RNC and the Node B is that: time of Node B=time of RNC+t2−t1−RTD. However, in the actual transmission process, the RTD and variation of the RTD should be considered, for the transmission delay in an interface Iub may not involve the actual situation of the transmission network.
With the equations above, if it is supposed that the transmission delay between the RNC and the Node B is almost invariable, the RNC can know the time of the Node B clearly with the granularity of the time be 0.125 milliseconds, i.e., 480 chip.
It is not enough to know the time of Node B for the RNC, it also clearly controls the transmission time of the common channel. As shown in FIG. 2, the transmission time of the common channel should be: BFN+Tcell+Soff. The following requirement should be satisfied to keep the time delays of different cells in transmitting data less than the required: an absolute value of ((BFN1+Tcell1−Soff1)−(BFN2+Tcell2+Soff2))<the required time delay.
In the WCDMA system, the transmission of a new copy of data can not be started from any of the frames and a transmission time interval TTI is defined for each transmission channel. The value of the TTI is integral multiple to the frame length, such as 10 milliseconds, 20 milliseconds, 40 milliseconds or 80 milliseconds and the corresponding number of frames occupied by each TTI is 1, 2, 4 and 8 respectively. The start point of these TTIs can only be the time when the CFN modulo Fn equals 0, where the Fn is the number of the frames corresponding to the TTIs. In addition, the RNC also controls the cells' or the common channel's configuration parameters so as to make the start point of the data received by the UE be the same within a TTI. Therefore the formula should be modified as: an absolute value of (((BFN1+Tcell1+Soff1)−(BFN2+Tcell2+Soff2)) mod Fn)<the required time delay.
Since the maximum lengths of Tcell and Soff are 9/15 milliseconds and 10 milliseconds respectively, it is impossible to make the time delay difference of data received from the two cells by the UE be less than 10 milliseconds in the case that the BFNs of the two cells are absolutely different, no matter how to regulate Tcell or Scoff. In general, the time of 10 milliseconds is greater than the time delay requirement for the UE in the signal merging. In the MBMS, the user data is transmitted through the common channel. To make all users receive the signal at any location of the cell, the required transmitting power of the common channel is higher. Therefore interference is generally caused to the adjacent cells so that the system capacity is reduced. Thus a method for merging the signals via the common channel of different cells has been proposed, which requires that the time delay of the signals transmitted from adjacent cells via the same common channel should not be over a certain value, otherwise, the object of signal merging and reception quality optimizing can not be reached. However, the time delay of the signals transmitted from adjacent cells via the same common channel can not be guaranteed within a relative small range in the current system at all.